Microporous membrane and method for providing the same

ABSTRACT

A method of producing a microporous membrane comprising the steps of: extruding a polymer at a temperature of from (polymer melting point +10° C.) to (polymer melting point +100° C.); drawing the extruded polymer at a rate of 5˜120 m/min in 10˜150° C. to obtain a polymer film; annealing the polymer film at a temperature of from (polymer melting point −100° C.) to (polymer melting point −5° C.) for 10 seconds to 1 hour; irradiating both surfaces of the annealed polymer film with an ion-particle amount of 10 2 ˜10 20  ion/cm 2  energized at 10 −2 ˜10 7  KeV, at an irradiating distance of 5˜100 cm under a vacuum of 10 −2 ˜10 −[ torr; cold stretching the irradiated polymer film at a temperature of from −20° C. to (polymer melting point −40° C.); hot stretching the cold stretched polymer film at a temperature of from (polymer melting point −40° C.) to (polymer melting point −5° C.); and heat setting the hot stretched polymer film at a temperature of from (polymer melting point −80° C.) to (polymer melting point −5° C.).

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a national phase application of InternationalApplication No. PCT/KR98/00365, which was filed on Nov. 16, 1998 andwhich published in English on May 27, 1999, which in turn claimspriority from Korean Application No. KR 1997/60661, which was filed onNov. 17, 1997.

BACKGROUND OF THE INVENTION

(a) Field of the Invention

The present invention relates to a microporous membrane and a method forproviding the same, and more particularly to a method for providing amicroporous membrane having hydrophilic/hydrophobic properties and poreswith uniform size and shape by irradiating energized ion particles to apolymer film under vacuum.

(b) Description of the Related Art

Currently, there are various types of microporous membranes being usedas a separator in lithium battery. Conventional methods for producingthese microporous membranes are classified into a wet method and drymethod. These methods utilize fillers or wax with a solvent as in wetmethod, or without the solvent as in dry method, to produce a precursorfilm. Then a resulting microporous membrane is obtained by formingmicro-pores in the precursor film.

There are numerous methods of forming micro-pores, such as in cold andhot stretching methods the precursor film is subjected to a stretchingprocess, and in an extraction method low molecular weight particles areextracted from the precursor film which has been subjected to a biaxialstretching (alternatively, biaxial stretching process can be implementedafter the extraction method) to form micro-pores on the precursor film.Further, the precursor film can be subjected to a corona dischargemethod followed by a stretching, or it can be etched after beingirradiated with high-energy ion-beams as in a track-etching method toobtain microporous membrane. The method utilizing cold or hot stretchingprocess is referred to as a dry process. U.S. Pat. Nos. 3,679,538;13,801,692 3,843,761; 4,238,459; and 5,013,439 disclose the dry process,while U.S. Pat. Nos. 3,471,597 and 3,880,966 disclose corona dischargeprocess for obtaining a precursor film with pores.

The dry process has an advantage in that it does not utilizeenvironmental hazardous solvents, and hence the method is referred to asa clean process and is widely used in the industry. However, microporousmembranes produced by the dry process have pores with undesirable smallsizes, and presents the difficulties of adjusting and increasing shapeand size of the pores. Further, there is a drawback in that duringstretching, maintaining shape of the pores becomes difficult as stretchratio increases.

The conventional methods for producing microporous mebranes to be usedas a separator in lithium battery utilize polyolefin resin because ofits cost and chemical and physical property. However, due to thehydrophobicity of the polyolefin resin, there is a low wettability ofelectrolytes for the separator. Currently, there are numerous researchesbeing carried out to incorporate hydrophilic property to polyolefinresin membranes. The method described by Hoechst Celenese processes thesurface of the polyolefin resin membrane with surfactants, and othermethods described by U.S. Pat. Nos. 3,231,530; 3,853,601; 3,951,815;4,039,440; and 4,340,482 integrates monomers having high hydrophillicproperty or processes the polyolefin resin membranes with chemicals.However, because of simultaneously occuring chemical reactions, themolecular weight of polymer decreases and the structural integrity ofthe polyolefin membrane weakens. Further, due to the complexity of theprocesses involved, it is difficult to mass produce the polyolefinmembranes having hydrophilic property.

Other methods for integrating hydrophilic property to the polyolefinmembranes are further described by U.S. Pat. Nos. 4,346,142; 5,085,775;and 5,294,346. These methods use monomers of acrylic acid havinghydrophilic property and polymers of polyethylene oxide by grafting themon to the surface of polymer membranes utilizing corona or plasmamethod. JP-A-8-31399 (unexamined published Japanese application)discloses a method of integrating both the hydrophilic and hydrophobicproperty to the polyolefin film surface by oxygen and carbontetrafluoride gas utilizing plasma or sputter etching method. However,due to the plasma's unique properties characterized by having a widerange of energy distribution and a high degree of environmentalsusceptibility, it is difficult to obtain an uniformed porosity.Further, obtaining a polyolefin membrane having excellent physicalproperties is made difficult by the degradation of its mechanicalproperty due to the damage to the surface of the film caused by thereactions accompanying the method.

SUMMARY OF THE INVENTION

In view of the foregoing, it is an object of the present invention toprovide a method for producing a microporous membrane havinghydrophilic/hydrophobic properties and pores with uniform size and shapeby irradiating energized ion particles to a polymer film under vacuum.

It is another object of the present invention to provide a method forproducing a microporous membrane having high-density of pores.

It is yet another object of the present invention to provide a simpleprocess method for producing a microporous membrane having hydrophilicproperty.

It is further object of the present invention to provide a method forproducing a microporous membrane having hydrophilic property andexcellent physical characteristics.

It is further object of the present invention to provide a microporousmembrane prepared by the method.

According to the above methods of the present invention, a microporousmembrane having excellent physical characteristics can also be obtainedby irradiating ion particles of the microporous membrane produced fromconventional methods.

DETAILED DESCRIPTION OF THE INVENTION

The present invention will be described in detail below.

The present invention utilizes the principles for decreasing contactangle of hydrophilic solvents onto the surface of a polymer surface andincreasing adhesion of the same by utilizing ion-beam irradiation.

Preparation of Precursor Film

A polymer film is obtained by using an extruder having a T-die or atubular die, and is made from a polyolefin group consisting ofpolypropylene, high-density polyethylene, low-density polyethylene, andlow-density linear polyethylene, because of cost and its low reactivity.Although an extrusion process can be carried out in a conventionalextruding temperature, it is more preferable to carry out the process intemperature range of (polymer film melting point +10° C.)˜(polymermelting point +100° C.). Extruding the polymer beyond this temperaturerange can lead to polymer degradation and consequently weaken itsphysical property.

The extruded polymer is drawn by using cast roll at 5˜120 m/min in10˜150° C. to obtain a precursor film, at draw down ratio of 10˜400 andthe freezing temperature of 10˜120° C.

Annealing

The precursor film is annealed at temperature range of (polymer filmmelting point −10° C.)˜(polymer melting point −100° C.) for 10 sec. to 1hour in order to obtain an elastic recovery over 40% at 25° C. Thisannealing process increases both the elastic recovery and crystallinityof the precursor polymer film. Annealing at a temperature higher thanthis range may melt the polymer film, and annealing at a temperaturelower than the range restricts the polymer movement and any significantincrease in both the elastic recovery and crstallinity is very marginal.

Irradiation

The annealed precursor film is placed in a vacuum chamber under10⁻²˜10⁻⁸ torr, then both surfaces of the precursor film were irradiatedwith an ion-gun. The ion-gun was prepared by injecting a gas forgenerating energized ion particles to be used in irradiation by changingelectrical current of ion-beam. Although, an irradiating distance fromion-gun to the surface of the precursor film of 5˜100 cm is adequate,irradiating distance should be adjusted according to the vacuum pressurein the chamber. Such that, the irradiating distance should be 15˜25 cmunder a high vacuum of 10⁻²˜10³¹ ³ torr, 25˜55 cm under a high vacuum of10⁻³˜10⁻⁶, and 55˜60 cm under a very high vacuum of 10⁻⁶˜10⁻⁷. Any gaswhich has the capability of generating ion particles can be used by theion-gun, however, electron, hydrogen, helium, oxygen, nitrogen, air,flourine, neon, argon, krypton, or N₂O and their mixture compounds arealso suitable for the purpose.

At this time, an energy level of the ion particles are set at 10⁻²˜10⁻⁷KeV and an irradiating amount is set at 10²˜10²⁰ ions/cm² by controllinga power supply device attached to the ion-gun. Irradiating ion particleswith the above energy level and the amount, a microporous polymer filmwas obtained.

During or after irradiating with the ion-beam, a reactive gas in theamount of 0.5-20 ml/min can be applied to the polymer film fordetermining hydrophilic or hydrophobic property of the polymer film,according to a type of reactive gas applied. For providing a polymerfilm having hydrophilic property, it is preferable to use helium,hydrogen, oxygen, nitrogen, air, N₂O, ammonia, carbon monoxide, carbondioxide, or methane or their mixture compound; and to provide a polymerfilm having hydrophobic property, it is preferable to use flourine,carbon tetraflouride or their mixture compound. This process ofdetermining hydrophilic or hydrophobic property of the polymer film canalso be carried out after a final microporous membrane has beenobtained.

Cold Stretching

The microporous polymer film obtained from the above irradiation processis subjected to a stretching process utilizing rolls or a biaxialstretcher by mono or biaxial stretching to increase the size ofmicro-pores formed in the polymer film. Here, the stretching isconducted at a temperature ranging from −20° C. to (polymer meltingpoint −40° C.).

Hot Stretching

The microporous polymer film stretched from the cold stretching processwas subjected to a further stretching process utilizing a roll or abiaxial stretcher by mono or biaxial stretching for obtainingmicro-pores of desired size having mechanical property. Here, thestretching is conducted at a temperature of from (polymer melting point−40° C.) to (polymer melting point −5° C.).

Heat Setting

The microporous film, hot stretched in a temperature below polymermelting point, having tension was then subjected to a heat setting undera tensioned state for maintaining the integrity of its stretched pores.Here, the heat setting is conducted at a temperature ranging from(polymer melting point −80° C.) to (polymer melting point −5° C.).

A microporous membrane produced by the above methods of the presentinvention having a circular or an elliptical shape and a pore size of0.005˜10 μm is suitable as a separator for lithium ion batteries.Additionally, a laminate membrane, produced by laminating a firstmicroporous membrane produced by the methods of the present inventionwith a second microporous membrane produced by the conventional methodswherein the irradiating step has not been utilized, is also suitable asa separator for a lithium battery.

The above process of the present invention describes and provides amethod for manufacturing a microporous membrane having excellentphysical properties. The steps of the process can be deleted, changed ormodified for providing a microporous membrane with a different or adesired property.

Herein below the preferred examples and comparative examples will bedescribed in detail.

EXAMPLE 1 A Polyethylene Microporous Membrane Obtained by UtilizingIon-beam Irradiation

A high-density precursor polyethylene film to a polyethylene microporousmembrane was obtained by utilizing polyethylene having a high-density of0.964 g/cc and a melt index of 0.3 g/10 min, a single screw extruderhaving a T-die, and a take-up device. In this process, an extrudingtemperature was set at 180° C., temperature of the take-up device's rollwas set at 110° C., and a draw speed was set at 35 m/min at a draw downratio of 70. The precursor film was annealed in a dry oven at 110° C.for 1 hour. The annealed precursor film was put in a vacuum chamberunder 10⁻⁵˜10⁻⁶ torr. Then, the precursor film was irradiated on bothsurfaces by an ion-gun with energized argon particles to formmicro-pores, and a polyethylene microporous membrane was obtained. Here,energy of ion-beam was set at 3 KeV, and irradiating ion particle amountwas set at 10¹⁸ ions/cm².

EXAMPLE 2 A Polyethylene Microporous Membrane Obtained by UtilizingIon-beam Irradiation and Cold Stretching

A precursor polymer film to a polyethylene microporous membrane wasobtained by the process described in Example 1, except the draw speedwas set at 30 m/min and draw ration was set at 60. Then, the extrudedprecursor polymer film was annealed and irradiated as in Example 1,except the energy of the ion-beam was changed to 1.5 KeV, and theirradiating ion particle amount was changed to 10¹⁷ ions/cm². Theprecursor film with micro-pores was then cold stretched in a machinerydirection (MD) at room temperature, to obtain a film with 150% of thelength of the respective film of Example 1 (herein after referred to asstretch ratio). Then, the stretched film was heat set under a tensionedstate using rolls at 115° C. for 2 minutes and cooled for obtaining apolyetheylene microporous membrane.

EXAMPLE 3 A Polyethylene Microporous Membrane Obtained by UtilizingIon-beam Irradiation and Hot Stretching

A polyethylene microporous membrane was obtained by the processdescribed in Example 2, except the energy of the ion-beam was changed to2 KeV, the irradiating ion particle amount was changed to 5×10¹⁷ions/cm², and the stretching temperature was changed to 115° C.

EXAMPLE 4 A Polyethylene Microporous Membrane Obtained by UtilizingIon-beam Irradiation and Both Cold and Hot Stretching

A polyethylene microporous membrane was obtained by the processdescribed in Example 2 except; a cold stretch method was carried out ata stretch ratio of 50% in room temperature; followed by a hot stretchingmethod was conducted at temperature of 115° C. and a stretch ratio of100%; the energy of ion-beam was changed to 1.5 KeV; and the irradiatingion particle amount was changed to 2×10¹⁷ ions/cm².

EXAMPLE 5 Obtaining a Polypropylene Microporous Membrane

A precursor polypropylene film to a polypropylene microporous membranewas obtained by a process described in Example 1. Here, polypropylene ofisotactic homopolymer having a density of 0.90 g/cc and a melt index of2.0 g/10 min was used. For this process, the extruding temperature wasset at 230° C., temperature of the take-up device's roll was set at 90°C., the draw speed was set at 40 m/min at draw down ratio of 80. Theprecursor film was then annealed in a dry oven of Example 1 at 140° C.for 1 hour. The annealed precursor film was irradiated on both surfaces,under vacuum, by an ion-gun with argon particles to form micro-pores asdescribed by Example 1. Here, the energy of ion-beam was set at 1.5 KeV,and irradiating ion particle amount was set at 5×10¹⁷ ions/cm². Afterirradiation, as described for Example 4, a cold stretching was conductedat room temperature at a stretch ratio of 30%, followed by a hot stretchmethod under 140° C. at a stretch ratio of 120%. Then, the stretchedfilm was heat set at 140° C. for 2 minutes and cooled to obtain apolypropylene microporous membrane.

EXAMPLE 6 A Microporous Membrane Manufactured fromPolypropylene/polyethylene Blend

A precursor polymer film was prepared from polypropylene andhigh-density polyethylene blend by the processes described in Examples 4and 5 utilizing the T-die and take-up device. The composition ratio ofpolypropylene/polyethylene was 70/30 by weight. In this process, theextruding temperature was set at 230° C., temperature of the take-updevice's roll was set at 85° C., and the draw speed was set at 40 m/minat draw down ratio of 80. The obtained precursor polymer film was placedin the dry oven of Example 1 at 120° C. for 1 hour to be annealed. Thenthe precursor polymer film was irradiated under the same conditions ofExample 1, and obtained micro-pores on the surface of the precursorpolymer film. Here, the ion-beam energy was set at 1.5 KeV and theirradiating ion particle amount was set at 2.5×10¹⁷ ions/cm². Afterirradiation, as described in Example 4, a cold stretching was conductedin room temperature at a stretch ratio of 30%, followed by a hot stretchmethod under 125° C. at a stretch ratio of 120%. Then, the stretchedfilm was heat set at 125° C. for 2 minutes and cooled to obtain apolypropylene/polyethylene microporous membrane.

EXAMPLE 7 A Microporous Membrane Manufactured from LaminatedPolypropylene/polyethylene

A high-density polyethylene precursor film and a polypropylene precursorfilm, each having thickness of 10 μm, were obtained by the processdescribed in Examples 4 and 5, respectively. The precursor films werepressed in polypropylene/high-density polyethylene/polypropylene orderedlayers at 130° C. under a pressure of 50 kg/cm². The laminate film wasirradiated under the same conditions and by the process described forExample 6, and micro-pores were formed on the laminate film. Then, thelaminate film with micro-pores was subjected to cold and hot stretchingprocesses, a heat setting process, and cooled as described in Example 6to obtain a microporous membrane.

EXAMPLE 8 A Polyethylene Microporous Membrane Obtained by UtilizingIon-beam Irradiation and Cold/hot(biaxial) Stretching

A high-density polyethylene precursor film, obtained by the process asdescribed in Example 4, was annealed. After annealing, micro-pores wereformed on the surface of the precursor film by irradiating under thesame conditions and by the process described in Example 4. Then, theprecursor film was subjected to a cold stretch as described in Example4, followed by a biaxial hot stretch process by utilizing a biaxialstretching device (manufactured by Toyoseiki Co. of Japan) at 115° C.Here, each stretch rotation X and Y, respectively, were set at a stretchratio of 100%. The film stretched at 115° C. was then heat set for 2minutes to obtain a polyethylene microporous membrane.

EXAMPLE 9 A Polyethylene Microporous Membrane Obtained by HotStretching, Ion-beam Irradiation, and Cold/hot (biaxial) StretchingMethods

A high-density precursor polyethylene film obtained from the processdescribed in Example 4 was subjected to a biaxial hot stretch process byutilizing a Toyoseiki biaxial stretching device at 115° C. Here, eachstretch rotation X and Y, respectively, were set at a stretch ratio of100%. The precursor film was annealed and then irradiated in the samecondition and by the process as described in Example 4 respectively, toform micro-pores on the surface of the precursor polyethylene film.After irradiation, the precursor film was cold and hot stretched, heatset, and cooled by the process as described in the Example 8 to obtain aresulting polyethylene microporous membrane.

EXAMPLE 10 A Polyethylene Microporous Membrane Having HydrophilicProperty Obtained by Utilizing Ion-beam Irradiation, Stretching

A high-density precursor polyethylene film obtained by the processdescribed in Examples 4 was annealed. After annealing process, theprecursor polymer film was irradiated under the same conditions ofExample 4. During irradiation, the surface of the precursor polymer filmwas treated with 4 ml/min of hydrophilic inducing reactive gas (oxygen)by utilizing gas injector device. Here, the ion-beam energy was set at1.0 KeV and the irradiating ion particle amount was set at 5×10¹⁶ions/cm². After irradiation, as described by Example 4, a coldstretching was conducted at room temperature, followed by a hotstretching at 115° C. Then, the precursor film stretched at 115° C. washeat set for 2 minutes and cooled for obtaining a polyethylenemicroporous membrane having hydrophilic property.

EXAMPLE 11 A Polyethylene Microporous Membrane Obtained by UtilizingIon-beam Irradiation, Stretching

A high-density precursor polyethylene film obtained by the processdescribed in Examples 10 was annealed. After annealing process, theprecursor polymeric film was irradiated under the same conditions ofExample 10 to form micro pores on the surface of the precursor film.During irradiation, the surface of the precursor polymer film wastreated with 4 ml/min of hydrophilic inducing reactive gas (oxygen) byutilizing gas injector device. Then, under the same irradiatingconditions, the other surface of the precursor polymer film was treatedwith 4 ml/min of hydrophobic inducing reactive gas (flourine). Afterirradiation, stretching methods were conducted as described by Example10. Then, the precursor film stretched at 115° C. was heat set for 2minutes and cooled for obtaining a polyethylene microporous membrane.

EXAMPLE 12 Increasing Hydrophilic Property of a Polyethylene MicroporousMembrane Obtained from Dry Process by Utilizing Ion-beam Irradiation andHydrophilic Inducing Reactive Gas

A high-density precursor polyethylene film obtained by the processesdescribed in Examples 4 was annealed. After annealing process,micro-pores were formed on the surface of the precursor polymer film bya conventional cold stretching method in room temperature. It was thensubjected to a hot stretching at 115° C., followed by heat setting.Here, the stretching were conducted by the process as described inExample 4, where the cold stretching and hot stretching were carried outat stretch ratios of 50% and 100%, respectively. Then, the precursorpolyethylene film was irradiated with ion particles and simultaneouslytreated with hydrophilic inducing reactive gas (oxygen) by the processesdescribed in Example 10. Here, the ion-beam energy was set at 1.0 KeV,the irradiating ion particle amount was 10¹⁶ ions/cm², and the reactivegas (oxygen) amount was 4 ml/min.

EXAMPLE 13 Increasing Hydrophilic Property of a Polyethylene MicroporousMembrane Obtained from Wet Process by Utilizing Ion-beam Irradiation andHydrophilic Inducing Reactive Gas

A precursor polyethylene film was obtained by utilizing the samehigh-density polyethylene of Example 1, ultra high molecular weightpolyethylene (UHMWPE, Mw of 2,500,000), and liquid paraffin by aconventional wet process. The composition ratio of high-densitypolyethylene:UHMWPE:liquid paraffin was set at 2:13:85 by weight. Asdescribed in Example 1, utilizing the T-die attached to the single screwextruder, the precursor film was obtained. In this process, theextruding temperature was set at 180° C., temperature of the take-updevice's roll was set at 30° C., and the draw speed was set at 5 m/minat draw down ratio of 10. The precursor film was then subjected to abiaxial hot stretch process by utilizing Toyoseiki biaxial stretchingdevice at 115° C. Here, each stretch rotation X and Y, respectively,were set at a stretch ratio of 100%. After stretching, liquid paraffinresidue was removed from the polyethylene film by methylene chloride orby compounds of hydrocarbons and chloride. Then, the polyethylene filmwas rinsed with water and dried. The resulting polyethylene film wasirradiated with ion particles and simultaneously treated withhydrophilic inducing oxygen gas by the process described in Example 12.

Comparative Example 1 A Polyethylene Microporous Membrane Obtained froma Conventional Dry Process

A polyethylene microporous membrane was obtained by the processdescribed in Example 12 without the steps of irradiating with ionparticles and treating with reactive gas.

Comparative Example 2 A Polyethylene Microporous Membrane Obtained byCorona Discharge

A polyethylene microporous membrane was obtained by the processdescribed in Example 4, except the surface of a precursor polyethylenefilm was additionally subjected to corona discharge process by utilizingvariable Tesla transitional coil at 12,000 Volts and at a rate of 1 in²of the film surface per 3 to 5 seconds.

Comparative Example 3 A Polyethylene Microporous Membrane Obtained by aConventional Wet Process

A polyethylene microporous membrane was obtained by the processdescribed in Example 13 without the steps of irradiating with ionparticles and treating with reactive gas.

Comparative Example 4 Increasing Hydrophilic Property of a PolyethyleneMicroporous Membrane by Plasma Treatment

The surface of a polyethylene microporous membrane obtained by the dryprocess of Comparative Example 1 was subjected to a plasma treatment atirradiating energy of 0.8 W/cm² and a pressure of 1 torr, in 30 secondsto obtain a microporous membrane with hydrophilic property.

From the above Examples 1 to 13 and Comparative Examples 1 to 4,membrane thickness, pore size, pore density, puncture strength,shut-down temperature, and melt-integrity values of microporousmembranes were obtained, and they are represented in Tables 1 to 3. Inaddition, air permeability, porosity, tensile strength, and tensilemodulus of microporous membranes were also obtained from the aboveExamples 1 to 13 and Comparative Examples 1 to 4, and they are alsorepresented in Tables 1 to 3. Further, water absorption rates ofmicroporous membranes obtained from the above Examples 10 to 13 andComparative Example 4 are represented in Table 3. Machinery direction(MD) indicated in Tables 1 to 3 represents a stretching direction andtransverse direction (TD) represents perpendicular direction of thestretching direction, in the present invention.

TABLE 1 Example 1 Example 2 Example 3 Example 4 Example 5 Example 6Thickness [μm]  23  24  24  24  25  25 Pore size [μm] 0.02 × 0.02 0.15 ×0.06 0.06 × 0.03 0.12 × 0.05 0.08 × 0.03 0.10 × 0.05 Porosity [%]  28 37  38  40  48  42 Pore density  18 × 10⁹  14 × 10⁹  14 × 10⁹  15 × 10⁹ 16 × 10⁹  15 × 10⁹ [pores/cm²] Air permeability 1270  980 1040  850 610725 [sec/100 cc] Puncture 380 304 340 295 510 470 Strength [g] Tensilestrength 1250/160  1300/165 1450/180 1410/200  1580/190  1530/180 (MD/TD) [kg/cm²] Tensile modulus 6500/3400 6620/3440 6800/3480 6850/34907200/3680 7000/3610 (MD/TD) [kg/cm²] Shut-down 126 128 127 128 161 130Temperature (° C.) Melt-integrity 135 134 135 135 167 167 Temperature (°C.)

TABLE 2 Comparative Comparative Comparative Example 7 Example 8 Example9 Example 1 Example 2 Example 3 Thickness [μm]  26  24  24  25 25  25Pore size [μm] 0.08 × 0.03 0.12 × 0.10 0.08 × 0.07 0.13 × 0.06 7 × 4 0.1× 0.1 Porosity [%]  45  42  41  37 50  36 Pore density  16 × 10⁹  15 ×10⁹  15 × 10⁹   6 × 10⁹   1 × 10⁹   5 × 10⁹ [pores/cm²] Air permeabilit690 625 650 960 18 670 [sec/100 cc] Puncture 520 335 364 260 95 440Strength [g] Tensile strengt 1590/200  1900/1300 2050/1500 1350/150 7000/50  1400/850  (MD/TD) [kg/cm²] Tensile modulus 7100/3670 6900/59807500/6120 6700/3470 5200/2740 6400/5630 (MD/TD) [kg/cm²] Shut-down 127129 129 131 132  129 Temperature (° C.) Melt-integrity 167 133 134 133133  138 Temperature (° C.)

As can be seen from the above Tables 1 and 2, the microporous membranesobtained by the process described in Examples 1 to 9 have more uniformpore sizes and higher pore density and the other physical propertiesthan the microporous membranes obtained by the process described inComparative Examples 1 to 3.

TABLE 3 Comparative Example 10 Example 11 Example 12 Example 13 Example4 Thickness  23  24  24  24  25 [μm] Pore size 0.02 × 0.02 0.15 × 0.060.06 × 0.03 0.12 × 0.05 0.08 × 0.03 [μm] Porosity  28  37  38  40  48[%] Pore  18 × 10⁹  14 × 10⁹  14 × 10⁹  15 × 10⁹  16 × 10⁹ density[pores/cm²] Air permeability 840 825 920 650 860 [sec/100 cc] Puncture290 283 287 428 230 Strength [g] Tensile strength 1250/160  1300/165 1450/180  1410/200  1580/190  (MD/TD) [kg/cm²] Tensile modulus 6500/34006620/3440 6800/3480 6850/3490 7200/3680 (MD/TD) [kg/cm²] Shut-down 128128 131 128 131 Temperature (° C.) Melt-integrity 134 134 133 138 133Temperature (° C.) Water absorption    2.2    2.3    2.6    2.5    5.3Speed [sec]

As it can be seen from the above Table 3, the microporous membranesobtained by the process described in Examples 10 to 13 have faster waterabsorption speed, higher hydrophilic property, and better physicalproperties than the microporous membranes obtained by the processdescribed in Comparative Examples 4.

According to the methods described in the examples of the presentinvention a microporous membrane having an uniformed pore size and ahigh pore density can be achieved by utilizing an ion-beam irradiation.Additionally, according to the methods disclosed by the presentinvention, a microporous membrane having a superior hydrophilc andphysical properties can also be obtained.

Further, according to the methods of the present invention a high drawdown ratio, a high elastic recovery, and a high crystallinity may not beconsidered as important requirements while overcoming drawbacks ofconventional methods.

In this disclosure, there is shown and described only the preferredexamples of the invention, but, as aforementioned, it is to beunderstood that the invention is capable of use in various othercombinations and environments and is capable of changes or modificationswithin the scope of the inventive concepts as expressed herein.

What is claimed is:
 1. A method of producing a microporous membrane,wherein the method comprises irradiating a polymer film with energizedion-particles under vacuum to form the microporous membrane, wherein thepolymer film is extruded at a temperature ranging from polymer meltingpoint +10° C. to polymer melting point +100° C., and the extrudedpolymer film is drawn at a rate of 5-120° C. m/min in 10-150° C.
 2. Themethod of producing a microporous membrane according to claim 1, whereinthe polymer film is irradiated on one or both surfaces.
 3. The method ofproducing a microporous membrane according to claim 1, wherein theenergized ion-particles are selected from the group consisting ofelectron, hydrogen, helium, oxygen, nitrogen, air, fluorine, neon,argon, krypton, N₂O and a mixture compound thereof.
 4. The method ofproducing a microporous membrane according to claim 1, wherein theirradiation is conducted under vacuum of 10⁻²˜10⁻⁸ torr at anirradiating distance of 5˜100 cm.
 5. The method of producing amicroporous membrane according to claim 1, wherein the ion-particleshave an energy of 10⁻²˜10⁷ KeV.
 6. The method of producing a microporousmembrane according to claim 1, wherein the irradiation is conducted withan energized ion-particles amount of 10²˜10²⁰ ions/cm².
 7. The method ofproducing a microporous membrane according to claim 1, wherein thepolymer film is subjected to an annealing process before and/or afterbeing irradiated.
 8. The method of producing a microporous membraneaccording to claim 7, wherein the annealing process is conducted at atemperature of from (polymer melting point −100° C.) to (polymer meltingpoint −5° C.) for 10 seconds to 1 hour.
 9. The method of producing amicroporous membrane according to claim 1, wherein the polymer film issubjected to an stretching process before and or after the irradiatingprocess.
 10. The method of producing a microporous membrane according toclaim 9, wherein the stretching process is conducted at a temperature offrom −20° C. to (polymer melting point −40° C.).
 11. The method ofproducing a microporous membrane according to claim 9, wherein thestretching process is conducted at a temperature of from (polymermelting point −40° C.) to (polymer melting point −5° C.).
 12. The methodof producing a microporous membrane according to claim 1, wherein thepolymer film is subjected to a heat setting process before and or afterthe irradiating process.
 13. The method of producing a microporousmembrane according to claim 12, wherein the heat setting process isconducted at a temperature of from (polymer melting point −80° C.) to(polymer melting point −5° C.).
 14. The method of producing amicroporous membrane according to claim 1, wherein during theirradiation process, a reactive gas is simultaneously applied to thesurface of the polymer film.
 15. The method of producing a microporousmembrane according to claim 14, wherein the reactive gas is selectedfrom the group consisting of helium, hydrogen, oxygen, nitrogen, air,ammonia, carbon monoxide, carbon dioxide, methane, fluorine, carbontetrafluoride, N₂O and a mixture compound thereof.
 16. The method ofproducing a microporous membrane according to claim 14, wherein thereactive gas is applied at an amount of 0.5˜20 ml/min.
 17. The method ofproducing a microporous membrane according to claim 1, wherein after thecompletion of the irradiation process, a reactive gas is applied to thesurface of the polymer film.
 18. The method of producing a microporousmembrane according to claim 17, wherein the reactive gas is selectedfrom the group consisting of helium, hydrogen, oxygen, nitrogen, air,ammonia, carbon monoxide, carbon dioxide, methane, fluorine, carbontetrafluoride, N₂O and a mixture compound thereof.
 19. The method ofproducing a microporous membrane according to claim 17, wherein thereactive gas is applied at an amount of 0.5˜20 ml/min.
 20. The method ofproducing a microporous membrane according to claim 17, wherein thepolymer film is a polyolefin selected from the group consisting ofpolypropylene, high-density polyethylene, low-density polyethylene,low-density linear polyethylene and a mixture thereof.
 21. A method ofproducing a microporous membrane having improved physical propertieswherein the method comprises irradiating a microporous membrane withenergized ion-particles under vacuum to form the microporous membranehaving improved physical properties.
 22. A method of producing amicroporous membrane having improved physical properties wherein themethod comprises injecting a reactive gas while irradiating amicroporous membrane with energized ion-particles under vacuum to formthe microporous membrane having improved physical properties.
 23. Amethod of producing a microporous membrane comprising the steps of: a)extruding a polymer at a temperature ranging from polymer melting point+10° C. to polymer melting point +100° C.; b) drawing the extrudedpolymer at a rate of 5-120 m/min in 10-150° C. to obtain a polymer film;c) annealing the polymer film at a temperature ranging from polymermelting point −100° C. to polymer melting point −5° C. for 10 seconds to1 hour; d) irradiating both surfaces of the annealed polymer film withan ion-particle amount of 10²-10²⁰ ion/cm² energized at 10⁻²-10⁷ KeV, atan irradiating distance of 5-100 cm under a vacuum of 10⁻²-10⁻⁸ torr; e)cold stretching the irradiated polymer film at a temperature rangingfrom −20° C. to polymer melting point −40° C.; f) hot stretching thecold stretched polymer film at a temperature ranging from polymermelting point −40° C. to polymer melting point −5° C.; and g) heatsetting the hot stretched polymer film at a temperature ranging frompolymer melting point −80° C. to polymer melting point −5° C. to formthe microporous membrane.
 24. A method of producing a microporousmembrane comprising the steps of: a) extruding a polymer at atemperature ranging from polymer melting point +10° C. to polymermelting point +100° C.; b) drawing the extruded polymer at a rate of5-120 m/min in 10-150° C. to obtain a polymer film; c) annealing thepolymer film at a temperature ranging from polymer melting point −100°C. to polymer melting point −5° C. for 10 seconds to 1 hour; d)irradiating both surfaces of the annealed polymer film with anion-particle amount of 10²-10²⁰ ion/cm² energized at 10⁻²-10⁷ KeV, at anirradiating distance of 5-100 cm under a vacuum of 10⁻²-10⁻⁸ torr; e)cold stretching the irradiated polymer film at a temperature rangingfrom −20° C. to polymer melting point −40° C.; f) hot stretching thecold stretched polymer film at a temperature ranging from polymermelting point −40° C. to polymer melting point −5° C.; and g) heatsetting the hot stretched polymer film at a temperature ranging frompolymer melting point −80° C. to polymer melting point −5° C. to formthe microporous membrane, wherein the polymer film is a polyolefinselected from the group consisting of polypropylene, high-densitypolyethylene, low-density polyethylene, and low-density linearpolyethylene.